Field of the Invention
The present invention relates to a centrifugal compressor, and more particularly, to a centrifugal compressor applied to a turbocharger for a vehicle.
Description of the Related Art
Turbochargers are typically used as superchargers for vehicles. A turbocharger drives a turbine using the energy of the exhaust gas exhausted from the engine, drives a centrifugal compressor coaxially coupled to the turbine, compresses gas (intake air), and supercharges the engine.
In such centrifugal compressors, there is a problem in that as the gas flow volume drops, flow reversal or laminar separation is produced in the flow of gas passing through the impeller, and a surge occurs. Consequently, there is a continual demand to expand the operating range by lowering the minimum allowable flow volume at which surging does not occur, or in other words, by improving the surge limit.
To improve the surge limit, Japanese Patent Laid-Open No. 2001-289197 describes an invention related to a centrifugal compressor having a circulating casing treatment. In times of low flow volume, static pressure is used to form a circulating flow that passes through hollow portions inside the casing near the leading edge of the impeller blades. Also, to further expand the operating range, Japanese Patent Laid-Open No. 2001-289197 describes exhausting from the hollow portions a circulating flow having a swirl component in the reverse rotational direction of the impeller.
Japanese Patent Laid-Open No. 2010-270641 describes a centrifugal compressor provided with an inlet guide vane that imparts to gas a swirl component in the opposite direction of the rotational direction of the impeller, on the downstream side of the exhaust port of the casing treatment and on the upstream side of the impeller.
However, in some cases, the layout of the gas channel on the upstream side of the impeller causes the flow of gas supplied to the impeller to have a swirl component in the direction of axial rotation of the impeller. In this case, with the device disclosed in Japanese Patent Laid-Open No. 2001-289197, it may become difficult to suppress surging with a circulating flow from the casing treatment.
With the device disclosed in Japanese Patent Laid-Open No. 2010-270641, the exhaust port of the casing treatment is farther upstream than the inlet guide vane. For this reason, it is difficult to suppress surging with a circulating flow from the exhaust port of the casing treatment.
Accordingly, the present invention has been devised in light of the above circumstances, and takes as an objective to provide a centrifugal compressor having a circulating casing treatment enabling an improved surge limit.